The invention relates to a backwashable filter,
in which backwashing is brought about by reversing the flow direction, PA1 having a layer of superimposed elements made from an elastically deformable material, PA1 whereof in each case two adjacent elements form a gap through which a flow takes place, PA1 whose inflow side located on one side of the superimposed elements is the accumulation point for contaminations by particles such as sand grains, etc. and PA1 whose outflow side is formed on the other side of the elements, PA1 which are kept spaced apart from one another with the aid of spacing members such as studs or lugs located on the elements PA1 whose opening edge forms the gap opening with the facing, fixed opening edge of the adjacent element, PA1 and that supports are positioned alongside the lip and whose facing side spaced from the lip allows a small movement of the lip in the flow direction until the lip strikes against said side, PA1 and the spacing of the opening edge of each lip from the facing, fixed opening edge of the adjacent element--starting from the position without loading by flow forces, on the path of the elastic deflection or elastic deformation of the lip decreases in the flow direction until striking the side of the support arranged alongside the lip, so that a particle jamming in the gap opening with the flow exerts a driving force on the opening edge of the lip which narrows the gap opening.
Backwashable filters have long been known. The elements normally comprise circular ring-shaped elements, which are superimposed as a layer for forming filter cartridges or candles. Filters of this type are e.g. used in steam condensers for filtering the cooling water or for conditioning the service water in the chemical and mechanical industries. The filter candles are generally placed on a filter plate, which has a number of holes corresponding to the number of filter candles. They form the access to the filter candles and consequently their filtering action occurs when there is a through-flow from the inside to the outside.
There are several possibilities available for the backwashing of filter candles. Firstly on the filter plate can rotate a suction shoe, which successively blocks access to each filter candle and with the aid of a suction pump places it under a vacuum or underpressure, whilst the filtering is continued in the remaining filter candles. In the filter candle subject to the suction pressure there is a reversal of the through-flow from the outside to the inside, so that the contaminants are rinsed away from the accumulation point and discharged from the candle. In place of a rotary suction shoe it is also possible to have a rotating filter plate, the suction shoe or shoes then being fixed. Diverging therefrom the filter plate can be subdivided on the inflow side into segments in the same number as the filter candles and each segment can be separately shut off and sucked off. There is then also a return flow of the filtered water from the outside to the inside through the filter candle and into the shut off segment and from there out of the filter housing, e.g. with the aid of a pipe.
German utility model 72 36 018 discloses a backwashable filter of the aforementioned type, in which the layerwise, superimposed elements form filter candles through which there is a flow from the inside to the outside. Each element comprises a ring or annular disk with inclined, movable tongues on the inside, which form pairwise, resilient, superimposed slits. The slits have on the inner edge of the tongues a relatively narrow opening cross-section and widen in the flow direction due to the inclined arrangement of the tongues. As a result of the elastic construction of the tongues they can be pressed apart during the backwashing process, so that the entrance cross-sections are correspondingly widened and contaminants jammed in the entrance area are freed during backwashing.
DE-OS 34 09 036 discloses a backwashable filter, in which the layer of superimposed elements in the form of filter candles is also formed from annular disks, through which there is a radial flow from the inside to the outside in filter operation. Two adjacent annular disks form a multiply subdivided annular clearance for the passage of the flow and which is constructed in such a way that the inner ends of in each case thinner annular disk portions are raised in the direction of a gap enlargement during the backwashing process from a thicker annular disk portion facing the gap spacing, so that any contaminants jammed in the annular clearance opening are freed during backwashing. The edge portion of the thinner annular disk consequently moves at right angles to the flow direction of the medium to be filtered or backwashed. This principle is also used in a block filter known from British patent 1 273 976. In place of the annular clearances there are rectangular gaps, whose one edge is formed by tongues, which can spring out transversely to the flow direction for gap enlargement purposes during backwashing.
European patent 159 961 discloses a backwashable filter, which has elastic spacers between the filter elements, as well as means able to vary the spacing between the filter elements by a corresponding change to the pressure exerted on the filter elements.
In the filter of the aforementioned type known from DE-OS 33 00 035 the annular disk-like, superimposed elements have inner Inclined and parallel spaced flanges. The flanges serve to deflect the flow, so as to improve the release of dirt particles which have collected in the gaps between the flanges during backwashing and bring about a prefiltering of the flow prior to its entry into the filter elements.
All the above-described filter systems are based on a common principle, according to which for influencing the size of the gap opening there is an elastic deformation of the elements bounding the gap openings transversely to the flow direction, i.e. transversely to the main direction of the forces exerted by the flow on dirt particles which have accumulated at the filter gap. As a result of these force conditions and the position and shape of the elastically deformable elements, dirt particles with dimensions approximately the same as the gap opening and particularly those which have wedge-shaped surface portions in the vicinity of the opening edges of the filter gap press away the elastic element bounding the latter by a wedge effect, so that the filter gap is widened accompanied by the building up of corresponding deformation forces and consequently dirt particles are Jammed in the gap opening with a specific clamping or Jamming force. During backwashing the Jamming forces securing the dirt particles can only be overcome if forces or deformations greater than the jamming forces are produced.
In the case of the filter known from European patent 159 961 for this purpose the external force acting on the stack of elements is reduced, so that the elements can be released from one another.
In the two other known filters the widening of the gap opening exclusively takes place through the described elastic deformation of the filter elements. This leads to the following relationships between the flow forces and the differential pressures at the gap opening during the filtering process and during the backwashing process. Shortly prior to initiating the backwashing process the filter has reached its greatest degree of contamination, the filter gaps are covered by dirt particles and the liquid to be cleaned flows at a higher speed and higher differential pressure through the still open residual gap opening. A 50% blockage of the gap opening increases the pressure difference to 400% compared with the clean filter and a 90% contamination leads to a 100 times pressure difference. As a result of the high differential pressures the dirt particles are forced with a very high force into the gap opening and jam, accompanied by the widening of said opening and as a result of the elastic deformation of the elements bounding the gap opening high jamming or retaining forces are built up.
During backwashing an increase in the size of the gap opening compared with the contaminated state is to be brought about. Therefore, in the vicinity of Jammed dirt particles there is a need for forces which are higher than the Jamming forces. As the gap opening at the start of the backwashing process can be freed immediately and without problem with respect to not Jammed dirt particles the return flow has a much larger gap opening available to it than shortly prior to backwashing. For the same water quantity flow there are consequently much smaller pressure differences and therefore much smaller forces than in the contaminated state.
Therefore the gap opening is not adequately widened, so that the Jamming of the dirt particles cannot be eliminated.
The negative effects of the pressure conditions and the strong clamping forces are further increased in that the long filter candles provided for large filter surfaces are subject to a lower velocity through-flow at their top during backwashing than at their bottom, so that the filter cartridges become irreversibly contaminated from the top until at the bottom a small residual surface is left which can still be cleaned by the backwashing flow.
In order to make difficult a widening of the gap opening in filter operation and a Jamming of dirt particles, attempts have been made to fit under pretension the elastic parts of the filter elements. However, this pretension must also be overcome for achieving a widening of the gap opening by the backwashing flow, so this merely leads to an increase in the problems during backwashing in the case of a pretensioning of the elastic parts.
Therefore the problem exists of so improving a backwashable filter of the aforementioned type that particles jammed in the gap opening can be more easily released than hitherto during backwashing, particularly at low flow velocities.